A number of photoelectric conversion devices generally used for detecting an electromagnetic wave are known, and for example, a photoelectric conversion device having sensitivity in ultra-violet rays to infrared rays is referred to as a light sensor in general. A light sensor having sensitivity in a visible radiation region with a wavelength of 400 to 700 nm is particularly referred to as a visible light sensor, and a large number of visible light sensors are used for devices which need illuminance adjustment or on/off control depending on human living environment.
In particular, in a display device, brightness in the periphery of the display device is detected to adjust display luminance thereof. This is done because unnecessary electric-power can be reduced by detecting peripheral brightness and obtaining appropriate display luminance. For example, such a light sensor for adjusting luminance is used for a cellular phone or a personal computer.
In addition, as well as peripheral brightness, luminance of a back light of a display device, particularly, a liquid crystal display device is also detected by a light sensor to adjust luminance of a display screen.
In such a light sensor, a photodiode is used for a sensing part and an output current of the photodiode is amplified in an amplifier circuit. As such an amplifier circuit, for example, a current mirror circuit is used (for example, see Patent Document 1: Patent Publication No. 3444093).